It is well known that organic dyes which absorb laser light can be used to mark substrates via a burning or “charring” mechanism. The absorption superheats the substrate causing localised charring and thus a contrasting image to form. Charring, however, can be highly damaging to substrates and thus is not suitable in all applications.
U.S. Pat. No. 6,911,262 teaches the use of organic near infrared absorbing dyes in laser welding applications.
U.S. Pat. No. 5,627,014 and its prior art preamble teaches the use of organic near infrared absorbing dyes in the conversion of organic leuco dyes into coloured compounds.
WO05/012442 teaches the use of conductive organic polymers as NIR absorbers in laser marking applications. However, conductive polymers when supplied as solutions are usually quite dilute (<2% w/w, ie weight per weight) and thus inconvenient to formulate with. Making them more concentrated can lead to instability such as gelling and flocculation. Conductive polymers also need to be used at either high or low pH, again this reduces formulation flexibility. The overall consequence of all these factors is a limited shelf-life stability of final ink formulations containing conductive polymers. When supplied in solid form conductive polymers can be very difficult to dissolve/disperse which also makes them arduous to use in ink formulations. Conductive polymers also usually have considerable visible absorptivity which gives them an undesirable dark colouration to the eye.